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The thermal stability of e-Ga 2 O 3 polymorph was studied by complementary methods. Epitaxial films of e-Ga 2 O 3 grown on c-oriented sapphire were annealed at temperatures in the range 700–1000 °C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC) up to 1100 °C was carried out on fragments of pure e-Ga 2 O 3 taken from a very thick layer. The results clearly indicate that e-Ga 2 O 3 initiates modifying its crystallographic structure above 650 °C, as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to β-phase occurs quite suddenly at 880–920 °C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 °C and rapid cooling the film is completely made of β-Ga 2 O 3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted β-Ga 2 O 3 layer assumes the standard orientation (−201) parallel to (00.1) of the Al 2 O 3 substrate. Based on the results of this study we conclude that e-Ga 2 O 3 may conveniently be used for device fabrication, exploiting its higher crystallographic symmetry and better matching to sapphire. However, all fabrication steps must be carried out at temperatures below 700 °C.

In this paper we report on the single stage deposition of CuInxGa1−xSe2 (CIGS)-based bifacial solar cells on glass coated with Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO) by single-stage low-temperature (250 °C) pulsed electron deposition (LTPED). We show that the mechanism of Sodium incorporation during the low-temperature deposition of CIGS on both FTO and ITO leads to the formation of a stable n+/p+ ohmic tunnel junction and photovoltaic efficiencies exceeding 14% can be obtained without any intentional bandgap grading of CIGS. The significant degradation of the cell fill factor with decreasing CIGS thickness is found to be related to the presence of craters left behind by micro-fragments of CIGS target, which are weakly incorporated in the film during the LTPED growth and removed during the subsequent process steps. Evidence is also presented that the low-temperature deposition of CIGS on ITO leads to the formation of a Ga-rich CIGS layer at the interface and to an unintentional compositional grading propagating towards the active region of the solar cells. The defects associated with this grading may be responsible for the loss in FF and Voc with respect to the cells deposited on FTO and Mo back contacts.

We present a comparative study of the symmetric metal-SI GaAs-metal (M-S-M) diodes with full-area contacts on both device sides, in order to demonstrate the effect of contact metal work function in a straightforward way. We compare the conventional high work function Pt contact versus the less explored low work function Mg contact. The Pt-S-Pt, Mg-S-Mg and mixed Mg-S-Pt structures are characterized by the current-voltage measurements, and individual Pt-S and Mg-S contacts are investigated by the X-ray photoelectron spectroscopy (XPS). The transport measurements of Mg-S-Pt structure show a significant current decrease at low bias while the Mg-S-Mg structure shows saturation current at high voltages more than an order of magnitude lower with respect to the Pt-S-Pt reference. The phenomena observed in Mg-containing samples are explained by the presence of insulating MgO layer at the M-S interface, instead of the elementary Mg, as confirmed by the XPS analysis. Alternative explanations of the influence of MgO layer on the effective resistance of the structures are presented. The reported findings have potential applications in M-S-M sensors and radiation detectors based on SI GaAs.

Recently some of the authors have proposed a new technique to deposit contacts on CdZnTe detectors that combines the standard electroless procedure and the scanning pipette technique. In this work it is shown that by using a 15 micron inner diameter alumina pipette it is possible to deposit gold dots, pixels and micro-stripes without using the standard photolithographic process. Moreover, it is shown that the performances in terms of charge collection efficiency and energy resolution of detectors prepared with the scanning pipette technique are similar to the ones of detectors prepared with the standard electroless procedure.

The low-bias current-voltage characteristics of surface barrier diodes, based on semi-insulating GaAs (SI-GaAs) with different contact metals and area, are reported and analyzed in order to demonstrate the possibility of tuning their low-bias transport characteristics. Novel applications of SI-GaAs emerge for devices with low-current at low-bias requirements, as follows from the possibility of current lowering by as much as two orders of magnitude, achieved by manipulation of the contact metallization and the contact area. The lowest current is observed in the structure with small Mg top and large-area Ti/Pt bottom contact. Since the observed behavior contradicts the conventional ohmic bulk limited and thermionic emission transport models, alternative explanations are discussed. The strong blocking ability of the low-work function Mg contact was attributed to the downwards band-bending, formation of a quasi-degenerate interface region accumulating charge carriers, and the corresponding lowering of the bulk SI-GaAs free carrier concentration. (C) 2013 Elsevier Ltd. All rights reserved.

Ge homojunctions were deposited by means of Metal Organic Vapour Phase Epitaxy (MOVPE) on Ge substrates at 550 degrees C and 675 degrees C, using AsH3 as n-type dopant. Ge-n/Ge-p, GaAsn/InGaPn/Ge-n/Ge-p and Ge-n/Ge-p/Ge-p structures were prepared and studied, where n and p identify the layer or substrate doping. Vertical mesa junctions were obtained on the above structures by using conventional photolithographic and evaporation techniques. The junctions were characterized by I-V measurements under dark and illumination conditions and by EBIC technique. It has been observed that the samples grown at lower temperature showed better rectifying I-V characteristics and light conversion efficiency while EBIC results may suggest that a high As diffusion is present in the samples grown at higher temperature. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A bipolar transistor, previously investigated as a possible radiation dosimeter and tested under industrial irradiation conditions in high-activity gamma and high-energy, high-power electron beam facilities has been subjected to stability test in order to understand its behaviour and help to improve its performances. Charge carrier lifetime was measured for several sets of transistors which were then irradiated with various doses (3–60 kGy): seven sets with 60 Co gamma rays and eight with a 10 MeV electron beam. After irradiation all the transistors were measured and each set was divided into three groups: one group was left untreated, the second group was heated at 100 °C for 30 minutes and the third group was heated at 150 °C for 30 minutes, for testing the stability of the lifetime. Our data showed that heat treatment quite successfully eliminates post-irradiation changes in the response. Response measurements of the irradiated transistors, heat-treated and untreated, were carried out at room temperature over several weeks after irradiation to establish post-irradiation stability and assess if these transistors could be used for recording dose history. Calibration curves in the range 3–60 kGy for the thermally treated and untreated devices are presented. Dependence of the response of the transistors on the temperature of the measurements in the range 20–50 °C is reported.

Localized states in Au/n-GaAs Schottky diodes, with InAs quantum dots (QDs) embedded in GaAs confining layers, were studied by means of low frequency noise measurements at temperatures ranging from 160 K to 299 K. Diodes containing a single array or three arrays of QDs were used; they all exhibited generation–recombination noise at low forward currents, which we attribute to local traps located in the GaAs layer. In the diode with a single array of QDs, a shallow trap level was detected with the activation energy about 0.037 eV, located above the Fermi level. In the diodes with three arrays of QDs we observed, in addition to the shallow level, a deep level located 0.1 eV below the midgap.

We report on GaAs samples containing an InAs quantum dot matrix grown from 3 monolayers of InAs. In this case, most of the mechanical stress relaxes by misfit dislocations. Capacitance measurements have been performed on macroscopic devices and, also, from a scanning microscope in which the capacitance is measured between the probe and sample surface. It was found that the electrical characteristics dramatically change during the capacitance measurements. This is explained by a degradation of the quantum dot layer which is attributed to the generation of point defects and/or dislocations. These results draw attention to the fact that, at the microscopic scale measurement, a small current may result in a large local current density which, in turn, degrades the device. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, basic tasks related to the spectrometric performance of X- and γ-ray radiation-imaging detectors based on semi-insulating (SI) GaAs are studied. The state of the art in this field is reported. The importance of a low-noise front-end read-out electronic chain is demonstrated. Four different charge-sensitive preamplifiers are tested and compared. The investigation of SI GaAs detectors considered for imaging applications is focused on the study of different electrode technologies. A blocking electrode is obtained by a surface Schottky barrier or an MBE-grown P + layer. An ohmic N + contact is fabricated using alloyed AuGeNi metallization or an advanced, quasi-ohmic non-injecting system created by a non-stoichiometric buffer grown by low-temperature MBE at the metal-semiconductor interface. This latter approach is considered as an improvement of the “non-alloyed” ohmic contact. The pulse height spectra obtained with 241 Am and 57 Co sources and corresponding detector energy resolution are evaluated. The role of detector geometry and temperature on the spectrometric performance is studied.

In this work, bulk semi-insulating (SI) InP wafers of four various producers have been used for the fabrication of radiation detectors. The tested detectors were prepared starting from the different materials in just one run in order to be sure that their performances were not influenced by technological processes. On one type of material various electrode technologies were used with the aim to analyze their role on the detector performances. The fabricated detectors were tested for detection performance by the 241 Am and 57 Co gamma-ray sources at below room temperature. The best detector was calibrated and tested also using 133 Ba and 137 Cs gamma sources. The best detector gives an energy resolution of 7 keV FWHM and a charge collection efficiency (CCE) of 82% (59.5 keV photopeak) at a temperature of 216 K. According to our knowledge, these results are the best which have been obtained with InP radiation detectors till now. The operation of SI InP detector at room temperature is demonstrated and discussed. In general, the CCE and the energy resolution of radiation detectors fabricated on bulk SI InP materials showed a strong dependence on the electrode technology as well as the base substrate quality.

High resistivity CdTe crystals were grown by Bridgman and by physical vapour transport techniques without intentional doping. Novel synthesis and pre-growth treatment procedures were developed for controlling background impurities concentration and stoichiometry of the source material. A detailed characterisation of the deep levels responsible for the high resistivity behaviour is reported.

The present work describes current–voltage ( I – V ) characteristics of radiation detector structures based on semi-insulating (SI) GaAs obtained with new types of metallizations used to form quasi-ohmic contacts. Three different metals (In, Gd and Mg) having lower work function in comparison with the standard AuGeNi eutectic alloy, were used. These metals are expected to form a band bending at the M–S interface which should give rise to a blocking barrier for holes (“minority” carriers in SI GaAs). Such contacts, which could effectively replace standard alloyed N + “ohmic” contact, show an unusual electrical charge transport as deduced from the measured I – V characteristics. Pulse-height spectra of 241 Am radionuclide source detected by the structures are also reported.

Hole capture at DX centers is evidenced in Schottky diodes made on Te-doped AlGaSb under forward bias conditions. Capacitance versus voltage measurements performed at low temperatures show that the occupancy of the DX centers is affected by sufficiently large forward biases. The current densities required to achieve such modifications are at least one order of magnitude smaller than those needed in Si-doped AlGaAs. Positive deep level transient spectroscopy signals are systematically detected in all the samples studied when forward-bias filling pulses are used. These observations are discussed in terms of minority carrier (hole) injection and subsequent capture by the DX centers.

Buried GaSb junctions were realized through Zn diffusion in Te-doped GaSb bulk crystals, the p-dopant being supplied through the epitaxial deposition of a heavily Zn-doped GaAs layer on GaSb. Structural and electrical investigations confirm the build-up of buried junctions, induced by Zn diffusion within GaSb, with satisfying rectifying properties. Surprisingly, the p-n junctions are formed more deeply with respect to the Zn diffusion front. A local rising up of the native acceptor density is assumed to drive the p-type conductivity conversion of the GaSb substrate beyond the Zn penetration depth.

High purity epitaxial 4H-SiC became a serious candidate for the fabrication of spectrometric radiation detectors with a high resistance to neutrons and gamma rays damage and suitable for applications in hot plasma diagnostics. The present work reports on i) the characterization of high purity epitaxial 4H-SiC grown by liquid phase epitaxy on SiC substrates and ii) the performances of metal/4H-SiC detectors fabricated on the same material. X-ray diffraction and topography as well as I-V, C-V and DLTS measurements are used for the evaluation of the material properties and device characteristics. The UV and X-ray detection abilities are evaluated by photocurrent measurements in the 3-6 eV region and pulse-height spectra measurements of the 241Am, respectively. Preliminary results of the detector hardness to fast neutron and gamma ray radiations are also reported.

It is shown that current instabilities, which are frequently observed at low temperatures in forward biased Schottky barriers on n‐type AlGaAs, result from changes in the distribution of negatively charged donors (DX centers) near the metal contact. These changes cannot be ascribed to sample‐heating effects, but they originate from hole injection in the barrier during forward biasing. The dominant mechanisms which are expected to induce the observed distortions in the DX center profile are (i) direct capture of the injected holes by the DX center and (ii) radiative electron‐hole recombination resulting in DX center photoionization. The role of the two mechanisms is discussed.

The role of quasistatic C‐V measurements in investigating DX‐center‐related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron‐capture rate by the DX center, at low temperature, is responsible for a frozen‐step‐like density profile of positively charged DX centers near the metal‐semiconductor interface. This causes a knee‐shaped 1/C2‐vs‐V plot and gives rise to an apparent built‐in potential. The low‐temperature freezing in of the free‐electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low‐temperature C‐V measurements performed on the sample cooled at different cooling rates.

We performed 2 MeV Fe implantation at a temperature of 200 °C on n-type InP substrates with different background doping concentrations. We studied the activation of Fe atoms as compensating deep acceptors and the electrical properties of the implanted layers. Simulation of the current–voltage characteristics coupled with secondary ion mass spectrometry depth profiling was used to extract important parameters such as the activated Fe fraction, the resistivity, and the thickness of the compensated layers. Our results show that resistivities of the order of 107 Ω cm can also be obtained for background doping concentrations higher than 1×1018 cm−3, with active Fe concentration well above the known solid solubility limit.

Different heterostructures, MOVPE grown by TBAs and TBP, lattice-matched to GaAs and containing a sequence of InGaP/GaAs quantum wells (QWs), were studied for the development of novel photovoltaic devices. Their structural characterisation exhibited a low structural and compositional disorder with a low defect density, also in the case of a consistent number (30) of QW periods. The optical properties, characterised. by excitonic features, show well resolved peaks ascribable to heavy and light hole to electron transitions, in good agreement with theory. The benefit role of the QW region was proved by an extension of the spectral response and a reduction of the reverse dark current. A main limitation to the conversion efficiency is discussed in terms of InGaP non-intentional impurity contamination. (c) 2007 Elsevier BN. All rights reserved.

In this work, the requirements of detector-grade semiconductor materials for radiation detectors, applicable in X-ray digital radiology, are identified. The study includes 12 various bulk semi-insulating (SI) GaAs single crystals grown by LEC and VGF methods, undoped and Cr-doped, obtained from 8 different suppliers. Conductivity, Hall, glow discharge mass spectrometry (GDMS), etch pit density (EPD), scanning electron beam induced current (S-EBIC), X-ray and laser scattering tomography (LST) techniques are used for the bulk SI GaAs material evaluation. The radiation detectors fabricated on these SI GaAs single crystals have been characterized by capacitance methods and their performances have been evaluated from detected pulse height spectra of 57Co. The correlation between the physical characteristics of the base materials and the performance of the detectors is demonstrated and discussed. Key detector-grade SI GaAs parameters, useful for material evaluation, are identified.

We present the study of optical and photoelectric properties of InAs quantum dots (QDs) grown on a metamorphic In0.15Ga0.85As buffer layer: such nanostructures show efficient light emission in the telecom window at 1.3 ?m (0.95 eV) at room temperature. We prepared a sample with vertical geometry of contacts isolated from the GaAs substrate. The structure is found to be photosensitive in the spectral range above 0.9 eV at room temperature, showing distinctive features in the photovoltage and photocurrent spectra attributed to QDs, InAs wetting layer, and In0.15Ga0.85As metamorphic buffer, while a drop in the photoelectric signal above 1.36 eV is related to the GaAs layer. No effect of defect centers on the photoelectrical properties is found, although they are observed in the absorption spectrum. We conclude that metamorphic QDs have a low amount of interface-related defects close to the optically active region and charge carriers can be effectively collected into InAs QDs.

Local trap levels in Au/n-GaAs Schottky diodes with embedded InAs quantum dots, generated after a long time of the device operation, have been investigated with low-frequency noise measurements performed in the temperature range of 77-298 K and at the forward current of 30 nA. Whereas the initial devices show a pure 1/f noise behavior, after a long time of operation, recombination noise was observed at frequencies above 100 Hz, in addition to the 1/f noise at lower frequencies. Analysis of the recombination noise data obtained on structures where different GaAs cap layer thicknesses have been removed by etching allowed us to determine the activation energy of the local traps and have a rough estimation of their spatial distribution.

This paper shows a comprehensive experimental and numerical investigation of proton-irradiated diodes for high-power snubberless applications. By means of DC and transient current–voltage measurements, OCVD extraction of lifetimes, C–V profiling, and DLTS trap characterization, a wide set of parameters was experimentally extracted and fed into a physically accurate mixed-mode simulation model. The numerical results are shown to be consistent with the available measured data, for example in showing the much better turn-off softness of proton-irradiated samples versus electron-irradiated ones. The complete physical/electrical model set-up in this work can now be used as an aid in the design and development of new proton-irradiated diodes.

In this work the role of electrode technology of a radiation detector based on Liquid Encapsulated Czochralski semi-insulating (SI) InP is investigated with emphasis on the development of a monolithic array of pixel detectors. Two different electrode technologies are applied: (i) Au evaporation to form a metal–semiconductor–metal structure with a quasi-Schottky barrier, and (ii) MOCVD of a p+ layer for the creation of a pin structure using the same SI InP base. The I–V characteristics and pulse height spectra of 241Am and 57Co are measured at room and lower temperatures and evaluated. The observed results show different electrical as well as detection performance of fabricated structures. Mechanical sawing and wet etched trench methods are applied with the aim of reducing electrical charge inter-diffusion between neighbouring strip detectors as a preliminary study to the fabrication of a segmented, monolithic detector array based on SI InP. SM and I–V methods are used for the evaluation of the wet chemical process used. The observed results and future works are discussed in view of results obtained within the study.

We present a study on deep levels in GaAs:Si grown by atomic layer molecular beam epitaxy (ALMBE). We show that in ALMBE GaAs grown at temperatures TS in the range 370–530 °C, the deep level concentrations: (i) are up to 3 orders of magnitude smaller than those of material grown by molecular beam epitaxy (MBE) at similar temperature ranges, and (ii) can be compared to those of GaAs grown by MBE at 600 °C. These features are independent whether Si is supplied during: (a) both the Ga and As subcycles, (b) the As subcycles, or (c) the Ga subcycles. Therefore, as for deep levels, ALMBE GaAs grown at 370≤TS≤530 °C can be related to GaAs prepared by MBE at conventional temperatures.

Due to the vanishing of both electron emission and capture rates, the DX centre shows a non-equilibrium occupancy at low temperature. It is shown that this effect results in a non-uniform free-electron density profile that can be controlled by biasing the junction during cooling. Preliminary results are reported showing that the I-V characteristics of Schottky barriers on AlGaAs (x=0.25) are affected by the DX centre occupation.

The electrical properties of GaAs/GaSb heterojunctions grown by metal-organic vapour phase epitaxy were carefully investigated. The structures were formed by heavily p(Zn)-doped GaAs layers deposited on n(Te)-doped GaSb bulk crystal used as substrates. The current-voltage characteristics showed the formation of a GaSb p-n homojunction, which was expected to be induced by Zn diffusion into GaSb. Nevertheless, secondary ion mass spectrometry pointed out a small penetration depth of Zn atoms in the GaSb substrate, resulting unaffected by post-growth annealing processes. Electron beam induced current analysis demonstrated that the p-n junction interface was located more deeply into the substrate (∼1 μm). This result was confirmed by capacitance-voltage (C-V) and electrochemical C-V characterizations. Admittance spectroscopy led to attribute the change of conduction type from n to p in GaSb to the formation of additional shallow acceptor levels, activated by GaAs growth and post-growth thermal annealing processes. An attempt to explain the formation of the buried junction in terms of atomic interdiffusion is provided, in order to justify, from the microscopic point of view, the low diffusivity of Zn in GaSb, and the apparently uncorrelated depth of the p-n junction interface in the substrate.

Al Schottky barriers have been prepared on n-In0.35Ga0.65As/buffer/GaAs structures using InxGa1-xAs buffers with different grading laws to reduce the defect density induced by lattice mismatch in the active In0.35Ga0.65As layer. Counter doped p+ cap layers have been used to enhance the barriers. Using suitable cap and buffer parameters, barrier heights up to 0.67 eV and ideality factors of 1.15 have been obtained.

A new procedure to profile deep‐level densities within the space‐charge region of Schottky barriers is presented. The method takes advantage of the spatial dependence of the time constant for the free‐electron capture by deep‐donor traps. The amplitude of the slow component of the capture capacitance transient following a negative reverse voltage pulse is simply related to the trap density at the point where the Fermi level crosses the trap level itself. The density profile of a given trap can be achieved by measuring the slow‐component transient signal as a function of the reverse voltage at a suitably chosen constant temperature. The estimated spatial resolution of the method was near 50 A in a practical case. Experimental density profiles for EL14, EL8, EL3, and the EC−ET =0.37 eV level in Cr/GaAs and Al/GaAs Schottky barriers are presented and discussed. The procedure is expected to be also applicable to the case of trap densities comparable with the shallow‐donor density without introducing large errors.

Cd1−xZnxS/p-GaAs heterojunctions for solar cell applications have been prepared by growing single crystal Cd1−xZnxS epitaxial layers on (111)GaAs substrates through a vapour phase chemical transport method using the close-spaced geometry and H2 as a transport agent. Electrical and photovoltaic properties of the heterojunctions have been investigated and discussed in connection with the main features of the growth technique. AM1 power conversion efficiencies up to 6.2% have been measured and possible improvements have been examined.

The small-signal a.c. admittance of a p +/n junction with gap states having a continuous distribution in energy is considered. Approximate analytical solutions for both diode conductance and capacitance are derived, through a space charge analysis, in the case of a low density of gap states. Numerical results on the zero-bias admittance are given and discussed for two particular energy distributions: i) Gaussian shapes and ii) exponential tails at the conduction band edge. It is shown that in both cases incomplete admittance spectroscopy experiments may lead to mistake the occurrence of a continuous distribution of gap states for an apparent discrete deep level. Unambiguous criteria to avoid misleading procedures are given and discussed.

GaAs layers grown by molecular‐beam epitaxy (MBE) have been postgrowth hydrogenated by implanting low‐energy hydrogen ions with a Kaufman source. Changes in the concentration of deep and shallow levels have been studied by DLTS and C–V measurements, respectively, for different hydrogen doses. A close match is obtained between the present results and those obtained for GaAs grown by the same MBE machine in hydrogen backpressure. Therefore, it is concluded that MBE growth with hydrogen results in hydrogen incorporation in the layers, and hence, in the passivation of shallow and deep levels.